The invention relates to the manufacture of monolithic integrated circuit (IC) structures of the kind that employ isolation diffusion to electrically isolate devices. In the typical IC, an epitaxial semiconductor layer is grown onto an opposite conductivity type wafer. Before such epitaxy, the active device regions are provided with doping of a conductivity type opposite to that of the wafer. This creates a high conductivity buried layer or insert under the epitaxial layer which acts to reduce parasitic resistance in the finished circuit. While the buried layers are created using slow diffusing impurity materials, they tend to expand into the epitaxial layer during subsequent high temperature processing, particularly the oxidation and diffusion steps. This, in turn, means that the epitaxial layer must be made relatively thick. Then the isolation diffusion must be made long enough for the impurities to completely penetrate the thick epitaxial layer. Since diffusion penetrates laterally as well as vertically, the isolation diffusion normally requires substantial surface area.